1. Field of the Invention
The invention relates to a kneading device that kneads a powder and a liquid.
2. Description of the Related Art
Recently, a lithium ion secondary battery has been used in a hybrid vehicle, an electric vehicle or the like. Each of electrodes in the lithium ion secondary battery is produced by kneading a powder of an active material (active substance) and the like in a solution of a thickener to produce a slurry of an active material, applying the produced slurry to a base material such as an aluminum foil, and drying the slurry.
The degree of dissolution of the thickener in the liquid solvent is important for performance of the secondary battery. The ratio of the mass of solute dissolved in a specific amount of solvent to the solubility (a maximum mass of the solute that can be dissolved in the solvent) is defined as a dissolution rate to solubility to be used in the description below. As an example of the device applicable to the kneading device, Japanese Patent Application Publication No. H11-90212 A (JP H11-90212 A) describes a device including a rotor and a stator. In the device, the rotor and the stator each have a stepped portion, and the stepped portions face each other. The stepped portions each extend radially outward in a concentric manner with a gradient in a radial direction. In the device, kneading can be carried out between the rotating rotor and the stator. Japanese Utility Model Application Publication No. H1-83192 U (JP H1-83192 U) describes a device including a pair of screws arranged side by side with their respective axes being parallel to each other. In the device, kneading can be carried out between the pair of rotating screws.
Japanese Patent Application Publication No. 2013-11594 (JP 2013-11594 A) describes a device including a pestle member and a mortar member. The pestle member has a distal-end outer periphery formed in a truncated-cone shape. The mortar member has a distal-end inner periphery that can contact the distal-end outer periphery of the pestle member and that is formed in a truncated-cone shape. The pestle member can be inserted in the mortar member. In the device, kneading can be carried out between the distal-end outer periphery of the rotating pestle member and the distal-end inner periphery of the mortar member.
The powder of the active material described above has low wettability with respect to the solution of the thickener. Thus, when the thickener, the solvent, the powder of the active material and the like are introduced at once and kneaded in the devices described in JP H11-90212 A, JP H1-83192 U, and JP 2013-11594 A, the powder of the active material may be damaged. In addition, demands for lithium ion secondary batteries have markedly increased, so it is desired to continuously perform the kneading. However, the devices described in JP H11-90212 A, JP H1-83192 U, and JP 2013-11594 A are devices for batch processing in which the thickener, the solvent, the powder of the active material and the like are introduced at once and kneaded. Therefore, it is difficult to use the devices in the continuous processing.
The device described in JP H11-90212 A is a device for kneading a hydrophobic liquid and a hydrophilic liquid between the rotating rotor and the stator so as to disperse the hydrophobic liquid in the hydrophilic liquid. Therefore, it is difficult to knead the powder of the active material having low wettability with respect to the solution of the thickener. Also, in the device described in JP H1-83192 U, when kneading conditions are changed, that is, when the gap between the pair of screws is changed, it is necessary to replace the pair of screws, which makes the changeover operation cumbersome. In the device described in JP 2013-11594 A, when the kneading conditions are changed, that is, the gap between the distal-end outer periphery of the pestle member and the distal-end inner periphery of the mortar member is changed, it is necessary to replace the pestle member and the mortar member, which makes the changeover operation cumbersome.